“LTE-Advanced,” which is an evolved version of Long Term Evolution (LTE), is currently under study in 3GPP. In LTE-Advanced, a heterogeneous network (HetNet) is under study as a technique for improving the frequency utilization efficiency or the like. The heterogeneous network disposes a cell (small cell) which is a base station having low transmission power such as a pico cell or femt cell in a macro cell in addition to a cell arrangement centered on the macro cell which is a conventional base station having high transmission power.
LTE-Advanced is proposing to separate, in a system constituting a heterogeneous network, a C-Plane (also referred to as “control plane”) from a U-Plane (also referred to as “data plane” or “user plane”) (hereinafter, referred to as “C/U separation”) (e.g., see Non-Patent Literature (hereinafter, simply referred to as “NPL”) 1). More specifically, during C/U separation, the macro cell performs movement management using the control plane to maintain connectivity and the small cell handles only the user plane using a wideband to thereby provide high throughput.
Furthermore, when the user operating environment is taken into consideration, it is expected that small cells are more likely to be arranged indoors. On the other hand, macro cells are expected to maintain connectivity in a wide range under the control of control planes. Thus, indoors where a small cell is disposed, there can be a situation where while a terminal (which may also be referred to as “UE”) can receive a user plane favorably, the terminal cannot receive a control plane. That is, there can be a situation where although it is possible to perform communication using a user plane, it is possible neither to perform communication using a control plane nor to maintain connectivity.
In contrast, NPL 1 discloses that both of a macro cell and a small cell include a control plane control section (RRC: radio resource control) and when it is not possible to maintain connectivity by the macro cell, the small cell transmits a control plane message to a terminal to thereby maintain connectivity. That is, the terminal continues communication even when it is outside a service area of the macro cell if it is located within the service area of the small cell and determines to be outside the communication range when links with both the macro cell and the small cell are outside the service areas.